The present invention relates to a power conversion apparatus including a switching element to be on/off controlled. More specifically, the present invention relates to a power conversion apparatus including a control circuit capable of achieving soft-switching of the switching element.
As proposed in U.S. Pat. No. 5,486,752, there has heretofore been known a PWM boost-up converter comprising an input reactor and a main switch connected in series with each other, wherein the input reactor and main switch are connected to a DC source and to an output terminal through an output diode, and the main switch is on/off controlled to obtain a stepped-up output, characterized by additionally including a serial resonant circuit composed of an inductor and a capacitor, and an auxiliary circuit composed of an auxiliary switch and an auxiliary diode to allow the main switch to be turned off at a zero current state, whereby a voltage surge can be suppressed to provide reduced turn-off loss. The same circuit is also described in the article titled xe2x80x9cNovel Zero-Current-Transition PWM Convertersxe2x80x9d, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol. 9, No. 6, pp 601-606, November 1994.
This circuit is constructed such that the auxiliary switch is turned on before turning off the main switch so as to pass a resonant current through the serial resonant circuit to conduct a diode connected in parallel with the main switch, and the main switch is turned off during a zero-current state yielded by the conduction of the diode. This circuit allows the main switch to be turned off at a zero-current state and thereby the voltage surge otherwise occurring at the main switch can be suppressed. This makes it possible to omit a snubber circuit and to achieve reduced turn-off loss, high efficiency and lowered noise.
However, in this conventional apparatus, a certain current is inevitably passing through the auxiliary switch when it is turned off, which undesirably causes a turn-off loss at the auxiliary switch. Further, if the main switch is turned on when a continuous current is passing through the reactor, a recovery current from the output diode passes through the main switch, resulting in occurrence of a turn-on loss and noise. Thus, the conventional circuit described in the aforementioned publications has suffered from limitations in enhancing efficiency and reducing noise.
It is an object of the present invention to solve the above problems in the PWM boost-up converter, specifically to provide a desirable switching timing control of the main and auxiliary switches, capable of achieving soft-switching in both the main and auxiliary switches, which allows switching-loss otherwise occurring at these switches to be reduced so as to provide high-efficiency, and allows voltage surge and current surge otherwise occurring in switching operations to be reduced so as to provide lowered noise.
The present invention essentially relates to a power conversion apparatus including positive and negative input terminals, positive and negative output terminals, an input reactor having one end connected to the positive input terminal, a main switch having one end connected with the other end of the input reactor and the other end connected to both the negative input terminal and the negative output terminal, a first diode connected in parallel with the main switch to have a forward direction from the negative input terminal to the positive input terminal, a main diode connected between the positive output terminal and the junction between the input reactor and the main switch to have a forward direction toward the positive output terminal, and a control circuit applied with a voltage between the output terminals as an input to form a switching signal for controlling an on/off operation of the main switch, wherein the main switch is on/off controlled according to the switching signal from the control circuit to generate an output.
In order to achieve the aforementioned object, according to the present invention, a snubber capacitor is connected in parallel with at least one of the main switch and the main diode. Further, there is provided a first auxiliary resonant circuit including first and second auxiliary switches connected in series with each other, a resonant inductor connected in series with the first and second auxiliary switches, and first and second auxiliary diodes connected in parallel with the first and second auxiliary switches, respectively, and the first auxiliary resonant circuit is connected between the negative input terminal and the junction between the main switch and the input reactor to have a forward direction of the auxiliary diode toward the junction between the main switch and the input reactor. Furthermore, a second auxiliary resonant circuit including third and fourth auxiliary diodes connected in series with each other is connected between the positive output terminal and the resonant inductor. A voltage detector is provided for detecting respective end voltages across the main switch and the auxiliary switches to generate voltage signals representing the respective end voltages and to input the end voltage signals to the control circuit. When a current from the input reactor is passing through the main diode before a turning-on signal is applied to the main switch, the control circuit is operable to apply a turning-on signal to the first and second auxiliary switches so as to turn on the first and second auxiliary switches to lead a current from the output terminals to the resonant inductor. Further, when a current passing through the resonant inductor is subsequently increased up to the same value as that of a current passing through the input reactor by a resonance generated in a resonant circuit formed of the resonant inductor and the snubber capacitor to provide approximately zero of the end voltage across the main switch, the control circuit is operable to apply a turning-on signal to the main switch.
In one embodiment of the present invention, there is provided an auxiliary-switch snubber capacitor connected between the junction between the first and second auxiliary switches and the junction between the third and fourth auxiliary diodes. Further, there is provided a voltage-detecting device for detecting a charged voltage of the auxiliary-switch snubber capacitor and for inputting a signal representing the charging voltage to the control circuit. When the charging voltage of the auxiliary-switch snubber capacitor is approximately equal to the voltage between the output terminals after the main switch is turned on, the control circuit is operable to apply a turning-off signal to the first auxiliary switch. Further, when the charging voltage of the auxiliary-switch snubber capacitor subsequently becomes approximately zero, the control circuit is operable to apply a turning-off signal to the second auxiliary switch.